Roof Membrane Wind Speed 2

[boomerz]

Hi,

I apologize if this is not in the correct forum. I was reviewing a number of roof membranes (TPO, PVC, EPDM) from various manufacturers and most specify that the membrane is warrantied up to wind speeds of 55 MPH. Does anyone know where this number came from and why it is used? Aside from coastal areas, ASCE-7 has wind speeds at 90 MPH in 2005 version, 115 MPH in 2010 version and it's just all over the place in the 2016 version. How do the suppliers verify that damage was done at wind speeds under "55 MPH" when locations vary and buildings are in different shapes and sizes?

Thanks in advance.

 

[JAE]

Check this discussion out:

http://bermanwright.com/single-ply-roof-membranes-and-common-design-and-warranty-considerations/

I think the issue is that roofs are typically warranted for shorter durations (20 or 30 years) vs. the return period for a 90 mph event (50 year).

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[MrHershey]

I think the issue is that roofs are typically warranted for shorter durations (20 or 30 years) vs. the return period for a 90 mph event (50 year).

Was thinking that myself, though if you look in Commentary to Appendix C in ASCE 7-10, the 10 year vs 25 year vs 50 year MRI wind speeds to bulk of the US don't differ by much at 76 mph vs 84 mph vs 90 mph.

 

[JAE]

Wonder if the "fastest mile" speed vs. the "3 second gust" speed is involved as well?



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[Ron]

There is a distinct disconnect between roof warranties and actual design conditions. Most roof system manufacturers only provide a warranty to wind speeds less than Category 1 hurricane force. The one you have mentioned goes even further....to 55 mph which is 3 mph less than Severe Thunderstorm category.

I have done millions of square feet of roof consulting and require that the manufacturer provide a warranty that matches the code mandated wind loads. They complain but most will comply if you push. They sell their products to comply with wind load criteria, either through UL or Factory Mutual tested ratings. They should then correspond their warranties to the same.

 

[dik]

They sell their products to comply with wind load criteria, either through UL or Factory Mutual tested ratings. They should then correspond their warranties to the same.

Updated my drawing notes to include... thanks,

Dik

 

[boomerz]

Thanks for the feedback.

It makes sense that the warranties only apply to lower wind speeds since it is only for a shorter 20 or 30 year duration. I found an article from the 2014 RCI Convention - Roofing Wind Speeds: ASCE 7, Uplift Ratings, and Warranties. It seems the use of wind speeds for single ply roof membrane warranties originated from the Beaufort scale. Any speeds above 55 MPH would be considered a storm (whole gale). However, some manufacturers used to use "fastest mile" and now "3-second gust" as well. This could certainly become confusing and challenging with all the different scales including the Saffir-Simpson hurricane scale.

It sounds to me like the best option is to have manufacturers provide a warranty that correlates to the design uplift loads per ASCE 7 or at least specify a wind speed using the 3-second gust in ASCE 7 since that is the basis of structural design.